In surgical operations, the attending physician is often faced with the problem of having to repair natural bone defects or bone defects occurring during the operation. The primarily utilized bone substitute materials are bio-inert or bioactive materials and are known by the terms "bio-ceramics, bio-glasses and bio-glass ceramics". Hitherto, "bio-inert" has been understood to refer to materials which do not trigger tissue reactions and do not give off any foreign substances. One can also include in this category titanium implant bodies, which have a layer of titanium oxide on the surface, which in this case performs the task of the bio-inert ceramic layer. In current usage, "bioactive" is used to denote materials which have the property of uniting directly with osseous tissue. These include bio-glasses and bio-glass ceramics which have on the surface considerable proportions of calcium phosphate ceramics, such as hydroxyl apatite ceramics and tricalcium phosphate ceramics. Consequently, as bone substitute material, various hydroxyl apatite ceramics and tricalcium phosphate ceramics are used, which are available in granulated form as well as in pre-fabricated moldings. The production of the materials requires very expensive sintering processes during which micro- and macroporosity can be achieved by temperature and certain additives.
To counter bone defects, it is also known to use implant materials based on polyacrylates (e.g., PMMA), which are filled with filler particles based on tricalcium phosphate (German Offenlegungsschrift 33 25 111). The Offenlegungsschrift also discloses that it can be advantageous to add further physiologically compatible resorbable materials in addition to the tricalcium phosphate. A porosity is produced on the surface of the substitute material due to the resorption and this is intended to facilitate the establishment of osseous material.
In a similar manner, German Offenlegungsschrift 27 52 297 discloses filled polymethyl methacrylate materials, which in addition to a resorbable filler (e.g. Na.sub.2 HPO.sub.4) contain carbonates and phosphoric acid. The interaction of phosphoric acid and carbonate produces a porous material structure upon admixing via a foaming process. Since, however, the materials contain liquid monomers and phosphoric acid, they are not toxicologically acceptable. In addition, high temperatures are to be anticipated during the setting reaction, during which adjoining tissue can be destroyed. In view of this, a "bioactive" effect is hardly possible.
For a considerable length of time, so-called "glass-ionomer cements" have been used in the field of dental medicine. These are reaction products of an aluminum fluorosilicate glass powder with a water-soluble polycarboxylic acid and water. These materials are used primarily for tooth-filling material and tooth-cement material; their use as bone cement, i.e., for fixing prostheses in bones, has also been described (see e.g. German Offenlegungsschrift 29 29 121).
German Offenlegungsschrift 27 36 447 discloses a process for the production of polymer ceramic materials from an ion-leachable glass powder and a polycarboxylic acid, in which the reaction components are not homogeneously mixed but sprayed on to a substrate surface in such a manner that the components are reacted with each other only after leaving the spray head. A blowing agent can be added to the two reaction components to provide foaming. The foaming that can be achieved with this spray process is not satisfactory for the present purposes, because the achievable total pore volume (V.sub.p) is too low. For the purposes of the invention it is necessary to provide for open-pored moldings which have interconnected pores and a total pore volume (V.sub.p) of at least 30% by volume, so that when the moldings are used as bone substitute material, the osseous material can grow right through the molded body without any difficulties and to an adequate extent. The materials disclosed in the Offenlegungsschrift are used primarily as building materials, e.g., as insulating construction bricks (see top of page 6).
German Offenlegungsschrift P 38 06 448 discloses porous moldings made from a workable material which contains no toxicologically unacceptable low-molecular monomers and which is bioactive. The porous moldings contain the product of the foaming and setting of (a) aluminum fluorosilicate glass, (b) at least one polycarboxylic acid with an average molecular weight of &gt;500, (c) a carbonate and/or hydrogencarbonate as foaming agent, (d) optionally a chelating agent and (e) water, and are suitable as bone substitute material.
The present invention relates to a further development of these moldings.